HTTP headers serve as a means to exchange contextual information between the client and server. But why is it essential to convey additional information with these requests or responses? The inclusion of headers in HTTP allows developers to fine-tune and optimize the communication process in various ways.
There is a large variety of http headers, but you’ll notice a select few showing up regularly when you keep an eye on your HTTP traffic. Common HTTP headers are used more frequently because they serve fundamental purposes applicable to a wide range of web interactions. Using headers such as “content-type” and “cache-control” optimize communication and align with programming best practices.
HTTP uses a request-response model for communication between clients and servers. The request allows clients to express what they want, such as retrieving a web page. The response provides the server’s answer, which could be the requested content or an error message.
Although HTTP headers often go unnoticed, they are an important component in the exchange of metadata between clients and servers. Metadata is the data that describes the actual data, it provides context behind the actual data being sent.
How to View Headers
- Open Developer Tools: Depending on your browser, you can typically open the developer tools by right-clicking on a web page and selecting “Inspect” or by pressing a keyboard shortcut like
Ctrl+Shift+I(Windows/Linux) in Google Chrome and similar shortcuts in other browsers.
- Navigate to the “Network” or “Inspector” Tab: Within the developer tools, find and select the “Network” or “Inspector” tab. This tab provides detailed information about network activity related to the webpage.
- Reload the Page: To capture network requests, you can either reload the page or interact with it.
- Select a Request: Click on a specific request in the list to view its details.
- Headers Section: Within the selected request’s details, you’ll find a “Headers” or “Request Headers” section. Here, you can expand these sections to see the HTTP headers associated with the request and response.
HTTP Packet Breakdown
When your web browser sends a request to load a webpage it begins an HTTP request-response transaction with a server. This transaction is comprised of two key components: the HTTP headers and the HTTP body. The headers are structured as plain text key-value pairs and appear right at the beginning of the request message. These request headers tell the server what the browser wants by containing details like the requested URL, the browser type, and the accepted content types.
After the server receives the HTTP message and handles the headers, it proceeds to process the body and prepare a response. This response also consists of headers and a body.
The response headers convey important information such as the status code and content type. The HTTP body holds the actual content, whether it’s the HTML of a webpage, a JSON data payload, or an image file.
There are numerous different http headers, but they can be effectively broken down into four categories based on context: request, response, representation, and payload. The two we have not talked about, representation and payload, may be present in both HTTP request and response messages.
Representation headers indicate how the data in the message is formatted, helping the client understand how to interpret the content. Payload headers deal with non-representation data, such as content length and encoding.
Common Request Headers
Host: Specifies the domain name of the server.
User-Agent: Provides information about the client making the request, often including details about the user’s browser or application.
Accept: Informs the server about the types of media or content formats the client can handle, typically used for content negotiation.
Accept-Language: Indicates the preferred languages for the response content, enabling the server to provide localized or language-specific content.
Accept-Encoding: Specifies the preferred content encoding methods, allowing the server to compress content for efficient transmission.
Referer: Contains the URL of the referring page, useful for tracking where a request originated.
Connection: Controls whether the connection should be kept alive for multiple requests or closed after a single request.
Upgrade-Insecure-Requests: Suggests that the client would like to upgrade to a more secure connection (HTTPS) if the server supports it.
Cache-Control: Provides directives for caching, specifying how the response should be cached and for how long.
Common Response Headers
Access-Control-Allow-Origin: Used in CORS to specify which origins are permitted to access a resource.
Connection: Indicates whether the connection will be kept alive for further requests or closed after this response.
Content-Encoding: Specifies the encoding applied to the response content, which can include compression methods like gzip.
Content-Type: Describes the media type and format of the response content, such as HTML, JSON, XML, or others.
Date: Provides the date and time when the response was generated, aiding in caching.
ETag: A unique identifier for the response, used for caching and conditional requests to check if the resource has changed.
Keep-Alive: Informs the client that the server supports persistent (keep-alive) connections.
Last-Modified: Indicates when the resource was last modified, facilitating conditional requests.
Server: Identifies the software and version running on the server.
Set-Cookie: Used to set cookies on the client’s side for session management and tracking.
Transfer-Encoding: Specifies the encoding or transformation applied to the message body, such as chunked encoding.
Vary: Informs caches whether the response is varied based on certain request headers, helping with cache optimization.
Common Representation Headers
Content-Type: Specifies the media type and format of the response content, such as “application/json” or “text/html.” It informs the client how to interpret the content.
Content-Encoding: Describes the encoding applied to the response content, such as “gzip” or “deflate.” It helps reduce the size of data for efficient transmission.
Content-Language: Indicates the language of the response content, allowing the server to provide language-specific content based on the client’s preferences.
Content-Location: Provides the URL or path of the resource, often used when the requested resource is a representation of another resource, allowing the client to discover the original resource’s location.
Common Payload Headers
Content-Length: Indicates the size of the response body in octets (8-bit bytes), helping the client know the length of the content being sent.
Content-Range: Used for partial content responses (HTTP status code 206), specifying the range of bytes included in the response body.
Trailer: Lists additional headers included in the response after the message body, often used when those headers cannot be determined in advance.
Transfer-Encoding: Specifies the encoding or transformations applied to the message body, such as “chunked” encoding, which divides the response into chunks for streaming.
Payload and representation headers are used to categorize header types and are not dedicated messages. They are used in both request and response messages, as evidenced by some headers appearing in duplicate roles.
There are many other HTTP headers available, and the reason for this diversity is to provide developers with flexibility and customization options for how they want the interaction between the client and server to occur. Developers can choose the headers that best suit their needs and use them to customize the behavior of their HTTP requests.